1. Field of the Invention
This invention relates to a projection type exposure apparatus used for the manufacture of integrated circuits, and particularly to an alignment apparatus thereof and an alignment method.
2. Description of the Prior Art
Generally, the process of making integrated circuits such as IC, LSI, etc. basically comprises the steps of resist application.fwdarw.alignment.fwdarw.exposure.fwdarw.development.fwdarw.etc hing repeated a plurality of times. The alignment mentioned hereinabove is a confirmation work for accurately maintaining a predetermined positional relation, during each exposure, relative to a pattern formed during the preceding exposure, and as the degree of integration of circuits becomes higher, the exposure pattern becomes finer and more accurate alignment is required.
An alignment method called the TTL alignment is known in which images of alignment marks on a reticle and a wafer formed through a projection objective lens by light of the same wavelength as the exposure light are observed in overlapped relationship. This alignment method, however, has a disadvantage that the resist on the wafer is exposed during alignment, and this has led to the necessity of limiting the exposed area as by a stop so that only the alignment marks are exposed during alignment. Also, if the alignment marks are exposed, the alignment marks disappear due to development in the case of a positive type resist, and this has given rise to various problems including the necessity of moving the alignment marks to the unexposed area each time.
An apparatus in which alignment is effected by the use of light of a wavelength different from the exposure wavelength has also been proposed. In the case of such apparatus, alignment light does not sensitize the photoresist and the above-mentioned problems do not occur. However, it is very difficult to completely aberration-correct the projection objective lens for both of the exposure wavelength and the wavelength of the alignment light, and it is difficult because of so-called chromatic aberration to maintain the accuracy of alignment high. Also, the apparatus has become complicated and has not been sufficiently satisfactory.